70 PHYSIOLOGICAL CHEMISTRY. 



with the exception of gelatin, so that the reaction is a nearly 

 universal one. The protein derivatives which still contain 

 the tyrosine complex likewise show the reaction. Tyrosine is 

 represented by the formula C H 4 OH.CH 2 .CHNH 2 .COOH, 

 and will be referred to later, as it is an important decomposi- 

 tion product of proteins. 



Ex. Test the behavior of Millon's reagent by adding some to white of 

 egg solution, milk or flour, and applying heat The characteristic color 

 appears almost immediately. Its depth depends somewhat on the concen- 

 tration of the protein substance used. Presence of much salt interferes 

 with the test or may even prevent the reaction. 



Ex. Apply the same test to weak solutions of phenol, salicylic acid and 

 thymol. Note the color and character of the reaction. These bodies all 

 have a benzene nucleus with hydroxyl combination. If pure tyrosine is 

 available a very dilute solution may be employed for tests. It is said that 

 I part in 1000 of water gives a distinct reaction. Hydroquinol, resorcinol 

 and a- and /3-naphthol give likewise decided reactions, but the colors are 

 orange yellow. 



The Biuret Reaction. This, like the above, is a protein 

 test depending on the presence of certain groups in the com- 

 plex molecule. When biuret and several substances of related 

 composition are mixed with an excess of alkali solution, either 

 sodium or potassium hydroxide, and then a few drops of a 

 weak copper sulphate solution are added, a blue-violet to red- 

 dish-violet color is produced. The shade depends on the con- 

 centration of the solution, and on the composition of the 

 reacting group. It has been shown that the reaction seems to 

 follow with compounds which contain two groups CONH 2 

 directly united or joined by a carbon or nitrogen atom, as for 

 example : 



CONH 2 /CONH 2 /CONH 2 CO-CONH, 



HN< H 2 C< | 



CONH 2 X CONH 2 X CONH 2 NH-CONH 2 



Oxamide Biuret Malondiamide Oxaluramide 



The combination of copper and alkali with these bodies has 

 been recently studied and formulas determined. If, in place 

 of using a solution of one of these compounds, some white of 

 egg or other protein solution is employed the same color 



